This invention relates to capacitors, and particularly roll-wound power capacitors, in which conductive interconnections are required between capacitor sections.
A capacitor unit for relatively high power applications normally comprises within a single enclosure a plurality of capacitor sections that are mutually interconnected in parallel and/or series combination to achieve the desired capacitance. The nature and method of forming the conductive interconnections between capacitor sections is the particular subject matter of this invention.
In conventional commercial practice the capacitor sections are formed by rolling on a mandrel sheets of dielectric material and electrode foil material. During the winding, such as by proceeding with winding a predetermined quantity of material and then stopping, a tab of conductive material is disposed in the winding adjacent or at least in conductive contact with one of the electrode foils. The other electrode foil is similarly contacted, usually at some other location in the roll. Various arrangements are used including those in which a single electrode foil has more than a single tab in contact with it. The tabs are disposed so as to extend from the completed roll, such as by several inches, to permit subsequent operations. A plurality of sections are stacked and pressed to form a compact group that is taped and wrapped as a group and disposed within the capacitor can or other enclosure. It has been necessary before the impregnation and sealing of the capacitor to perform operations to interconnect the various conductive tabs with each other to form the desired conductive interconnections among the sections. Past practice has entailed the forming of a physical connection between tabs using thermal bonding techniques such as soldering or mechanical techniques involving various forms of connectors. These have generally been successfully performed but involve expense and, in the case of soldering, some operator skill in successfully performing the interconnections without damage to other elements of the capacitor.
In particular, for example, power capacitors frequently employ as one of the dielectric materials a plastic film, commonly polypropylene. While polypropylene has long been successfully used in power capacitors, it is recognized that thoroughness of impregnation by the dielectric liquid that fills the capacitor is desirable. This impregnation is facilitated in some structures by accompanying the polypropylene film layer by one or more layers of capacitor grade paper of an inherently more porous nature which provides a wicking action to permit entry of impregnant into the interior of the capacitor sections. There is present interest in providing all film capacitors in which the paper layers are not used. This can impose some extra difficulty in the impregnation step. The nature and manner of making the interconnections between sections can affect the ability to perform thorough impregnation, particularly in an all film unit. When soldered interconnections are formed, the soldered tabs are necessarily very close to the end of the capacitor section rolls themselves. The soldering in this area can, if not very carefully performed, cause heating of the end of the capacitor roll to the extent that a film dielectric material such as polypropylene will melt or fuse. The degree of this form of damage is not at all likely to be substantial in terms of the overall capacitor, but impregnation can be influenced because the fusing itself tends to seal the end of the section and will inhibit the entry of fluid dielectric therein.
It is therefore for the purpose of improved reliability as well as economy that the instant invention in the formation of interconnections between sections of a multi-section capacitor came about.
By the present invention, the conductive elements inserted in the roll include two general forms. One form is a conductive sleeve which may be and preferably is merely a sleeve of folded conductive foil material generally like that of the electrodes. The other of the interconnection conductive materials are simply conductive tabs that are located in other positions in these capacitor structures. By the present invention the section rolls including such elements at the desired locations, in accordance with past practice or any desired configuration for the interconnections, are stacked together and, where desired, the conductive tab free end is merely inserted into the extending conductive sleeve, in another section. The sections are then pressed so that among other things the conductive elements remain tightly contained, the sections are taped and wrapped as a group and placed within their enclosure.
Along with the ability to make interconnection between sections as described is that of making solderless interconnections within a section. That is, where desired to "short circuit" two points of the same electrode foil, such as to achieve lowered electrical power losses or lower inductance, the tab and sleeve in accordance with this invention may be used.
By this technique, soldering or an other form of physical interconnection is not required. The connection is formed by a continuous conductive tab extending from one location in a capacitor section to another.